I personnaly think that Bob is right.

The problematic of religion in war in mainly a political question and has very little to see with what combattants do or not.
Looking at war laws (1907) and humanitarian laws (1949), they all tend to be secular with a "religious" base as they tend to bring moral and good behaviour in war. (Early laws or code of conduct from ancient India and Mesopotamian in particular.) But the fact that you believe or not does not really matter in their application nowadays except at individual/personal level.